Zend certified PHP/Magento developer

Stop Firefox from suggesting site-specific search engines

I would like to disable Firefox’s feature that suggests adding the search engine for the site I’m currently on to my available search engines. For example, if I click on the magnifying glass right now with the green plus on it, it will offer the option to “Add Super User.”

Example screenshot from Mozilla’s support page:

Firefox Search Engine Suggestion Screenshot

How can I turn that off so I don’t keep getting that green plus on every site I visit?